1. Field of the Invention
The present invention relates to an image pickup lens which is mounted on a relatively small-sized camera such as a scanner, a copying machine, a network camera and the like, and which forms an image of an object on an imaging element such as a CCD sensor, a CMOS sensor and the like. Further, the present invention relates to an image pickup lens applicable to a security camera, a car-mounted camera, a game machine, a digital still camera, and a camera embedded in a mobile device and the like.
2. Description of the Related Art
Accompanying the popularization of portable phones, information terminals and smartphones having the function of a computer, performance of built-in cameras have improved, and the resolutions thereof now match that of digital still cameras. An image pickup lens mounted on the portable phone, smartphone and the like is required to have an optical performance with high-resolution and also to be downsized, so that it is required to have optical characteristics equipped with a favorable aberration correcting ability satisfying both needs.
On the other hand, application of the image pickup lens to a scanner, a copying machine, a network camera and the like has been considered recently. In the case where the image pickup lens is applied to the scanner, the copying machine, the network camera and the like, the lens is required to have wider angle in addition to high-resolution. Further, compared to the aberration correcting technique of a conventional typical image pickup lens, even higher aberration correcting technique is required. Especially, a technique of correcting distortion with high precision to even the circumferential edge portion of an imaging surface is required.
Methods for achieving widening of angle, such as making a focal length of the image pickup lens shorter, setting an entrance pupil near an image side principal point, and the like, are known. Conventional arts for realizing widening of angle and high-resolution are listed below. Japanese Patent Laid-Open No. 2011-100094 (Patent Document 1) and Japanese Patent Laid-Open No. 2011-209677 (Patent Document 2) disclose an image scanning lens as an area sensor, in place of a line sensor. The image scanning lens achieved a half angle of field of 44.8° to 53.45°, a total track length of 8.8 mm to 14.96 mm, and Fno of 2.88 to 3.00, however, it has a problem that the distortion increases in the range of approximately 70% position and above of an image height to a maximum image height. Further, it has a problem of insufficient correction of chromatic aberration of magnification. An image scanning lens disclosed in Japanese Patent Laid-Open No. 2009-204997 (Patent Document 3) has a half angle of field of 39.7° to 40.1°, a focal length of 31.0 mm to 31.98 mm, and a total track length is long at 24.0 mm to 42.3 mm. The lens achieved a distortion of ±1.0%, and a chromatic aberration of magnification of ±10 μm, however, it is not suitable for obtaining bright image since Fno is large at 4.5. Japanese Patent Laid-Open No. 2007-121743 (Patent Document 4) has a half angle of field of 32.7° to 33.0°, a focal length of 23.4 mm to 23.6 mm, and achieved a distortion of ±1.0%, however, Fno is large at 7.0. Japanese Patent Laid-Open No. 2005-018041 (Patent Document 5) has a half angle of field of 31° to 35°, a focal length of 4.30 mm to 4.90 mm, and Fno of 2.0 to 4.0, realizing a comparatively wide-angle and bright lens system, however, a distortion near 70% position to 80% position of an image height is relatively large as 2.0% to 3.5%. Further, Japanese Patent Laid-Open No. 2007-122007 (Patent Document 6) has a half angle of field of 29.5° to 37.3°, a focal length of 3.8 mm to 5.57 mm, and Fno of 2.8 to 3.0, realizing a comparatively wide-angle and bright lens system, however, a distortion in 20% position to 70% position of an image height is relatively large as 1.5% to 2.3%, and correction thereof is insufficient.
In Patent Documents 1 through 6 mentioned above, it is difficult to obtain the image pickup lens with comparatively bright image, high-resolution with various aberrations corrected satisfactorily, in particular the distortion suppressed small, and which corresponds to widening of angle. The present invention aims at providing an image pickup lens which is bright, compact, with distortion corrected satisfactorily, and has a relatively wide angle of field.